1. Field of the Invention
This invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to potentiate fusion, and more particularly to an implantable device having superior lateral stability and overall strength.
2. Description of the Prior Art
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes which can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art which achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Referring now to FIGS. 1 and 2, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages 10 generally comprise tubular metal body 12 having an external surface threading 14. They are inserted transverse to the axis of the spine 16, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in FIG. 2 the pair of cages 10 are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages 10 are generally inserted side by side with the external threading 14 tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) below. The cages 10 include holes 18 through which the adjacent bones are to grow. Additional material, for example autogenous bone graft materials, may be inserted into the hollow interior 20 of the cage 10 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage 10.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height, however, they do have specific drawbacks which limit their effectiveness. First among these drawbacks is that the devices, once implanted, do not permit the spine to retain its original and proper curvature. Causing a fusion to grow and immobilize the spine at a curvature which is not natural can cause discomfort and potentially damaging effects.
A second concern with respect to cylindrical implants of this type is that there will be a tendency for the devices to roll and/or slide. Such undesirable motion by the implant can cause loosening, or worse, complete dislocation from its proper position.
A third limitation of these devices is that the overall volume for true bone fusion growth is limited by the geometry of the tubular device, i.e., much of the fusion must grow through the tubular section itself. Much issue has been taken with the fact that dependence upon fusion growth within the device at a microscopic level is not sufficient to provide the true stability and immobilization necessary for total healing of the site.
It is, therefore, an object of the present invention to provide a new and novel vertebral/intervertebral spacer which conforms to the natural curvature of the patient's spine.
It is further an object of the present invention to minimize the risk of dislocation by providing a geometric shape which is more suitable for stable positioning than cylindrical implants.
It is further an object of the present invention to provide a device for implantation into the intervertebral space which provides greater volume external to the device for bone fusion growth.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.